Substrate comprising an abrasion-resistant diffusion barrier layer system

ABSTRACT

A substrate has an abrasion-resistant diffusion barrier layer system having: a hard base layer including a coating composition based on a polymer containing reactive surface groups; and a nanostructured topcoat obtained by applying a nanoscale composition comprising sol particles and/or particulate solids to the basecoat and then curing it. The system features good abrasion resistance and diffusion barrier effect, and is particularly suitable as a protective coat for plastic substrates.

The present invention relates to a substrate provided with anabrasion-resistant diffusion barrier coat system, the coating systemcomprising a hard basecoat and at least one nanostructured, i.e.nanophase-containing, topcoat, and to a process for producing thesubstrate provided with an abrasion-resistant diffusion barrier coatsystem.

The coating of the substrates with coats having particular physicalproperties, such as high refractive index, high abrasion resistance,protection against the inward diffusion of substances, e.g. gases fromthe atmosphere or water, for example, is a great problem particularly inthe case of substrates unable to withstand thermal loads, since withoutthermal densification to give purely inorganic coats, a hermetic seal isextremely difficult to achieve. Even with coats applied by sputtering,the frequency of pinholes and other defects is such that there can be notalk of a hermetic seal. In many cases, however, coats of this kind areadequate; although hermetic sealing is not achieved, the surfaceobtained is sufficiently sound.

Additionally, coatings obtained by wet-chemical means, in which coatingtakes place from a solvent phase, lead to structures which are generallyvery open (polymers having high free volumes), and in systems containingparticles there are likely to be interstices through which gas diffusionand material transport may take place. Consequently, it is not possiblewith these systems either to obtain an effective seal withoutdensification at high temperatures.

Even inorganic/organic composite systems are unable to provide adequatesealing without additional inorganic sputtering coats. The abrasionresistance of such coats, although much greater than that of typicalplastics, such as PC and PMMA, is nevertheless inadequate for manypurposes (for example, glazing systems).

Thin coats of less than 1 μm can generally be produced by gas phasedeposition processes but also by sol-gel processes. With gas phasedeposition processes it is possible to produce even purely inorganiccoats. Purely inorganic coats, if they can be produced without pores,seal off the substrate hermetically even in very thin coats; substancessuch as gases or water are unable to enter by diffusion. This factor isconnected with the density of the network, which does not possess thefree volumes that are the case with organic polymers. Furthermore, theylack sufficient flexibility to allow passage of gas molecules.

Following the deposition process, inorganic sol-gel materials have arelatively low theoretical density—in other words, there is no closepacking, since a high packing density is prevented by the interaction ofthe sol particles and/or sol molecules. Interaction comes about throughdipolar interactions and/or hydrogen bonds or chemical bonds, andprevents relaxation taking place at low temperatures. The typicalpacking densities of such coats are situated at between 5 and 25% of thetheoretical density.

Whereas subsequent densification is possible by using high temperatureswhen such coats are applied to ceramic and vitreous materials, it is notpossible in the case of application to polymer substrates. Typicaldensification temperatures of inorganic systems are situated at between450 and 1000° C., and such processes are therefore unsuitable forpolymers. Additionally, although it is possible to obtain relativelythin coats on polymer substrates with the abovementioned systems, theresulting coats are of extremely low mechanical stability and lowscratch resistance. It is a problem in particular for transparent coatswhich are used in optics, since in such applications the requirement isoften for very thin coats.

The object of the invention was therefore to provide anabrasion-resistant diffusion barrier coat system comprising a system ofthin coats with high mechanical strength and an increased diffusionbarrier effect. The intention in particular was to achieve this withoutthe need for a heat treatment at high temperatures (for example,densification of the coats at from 450° C. to 1000° C.), so that thecoating system is suitable even for substrates which cannot be exposedto such high temperatures. A further intention was that transparentcoats should also be possible, so that coated substrates having theaforementioned properties can be obtained that are suitable for opticalapplications. The coated substrates, moreover, ought to be obtainable byway of a wet-chemical process.

The object of the invention has surprisingly been achieved by asubstrate having an abrasion-resistant diffusion barrier coat systemwhich comprises

-   -   a hard basecoat comprising a coating composition based on        compounds which are polymerizable or curable thermally or        photochemically to form a polymer, and    -   a nanostructured topcoat obtainable by applying a composition        comprising nanoscale sol particles and/or particulate solids to        the basecoat, still containing reactive surface groups, and then        carrying out heat treatment or curing.

The hard basecoat comprises a coating composition based on compoundswhich are polymerizable or curable thermally or photochemically to forma polymer. The curable or polymerizable compounds comprise inorganiccompounds, organically modified inorganic compounds, or purely organiccompounds or monomers, it being also possible, of course, to usemixtures thereof. Preference is given to using organically modifiedinorganic compounds or mixtures of organically modified inorganiccompounds and inorganic compounds, in the latter case the amount oforganically modified inorganic compounds being preferably at least 40mol %, with particular preference at least 60 mol %. Overall, preferablyat least 20 mol %, with particular preference at least 40 mol %, of allof the polymerizable or curable compounds used are organic compoundsand/or organically modified inorganic compounds.

The term polymerization as used here is intended to embrace allcustomary polymerization reactions, such as free-radical additionpolymerization, polycondensation or polyaddition. This embraces inparticular the (poly)condensation of the hydrolysable compounds thattakes place in the context of the sol-gel process elucidated later onbelow. The resulting condensates, accordingly, are likewise polymers. Bycuring (crosslinking) is meant in particular the process of linking toform a three-dimensional network. This embraces the condensation ofhydrolysable compounds to form a three-dimensional network. In thecoating composition, the compounds may be present as monomers or elsemay comprise oligomers or (pre)polymers which have already undergone atleast partial polymerization or crosslinking. In the coatingcompositions which include inorganic compounds or organically modifiedinorganic compounds, these compounds may then be present, for example,in already partly hydrolyzed and/or condensed form.

The coating composition based on the compounds which are polymerizableor curable thermally or photochemically to form a polymer is preferablya coating composition based on compounds of glass-forming and/orceramic-forming elements. These compounds are, in particular,hydrolysable and condensable compounds. The coating composition isobtained from these compounds preferably from the sol-gel process.Examples of glass-forming and/or ceramic-forming elements are theelements of groups 3 to 6 and 12 to 15 of the Periodic Table, or thelanthamide elements.

These elements preferably comprise Si, Al, B. Pb, Sn, Ti, Zr, V and Zn,especially Si, Al, Ti and Zr, or mixtures thereof. It is also possibleto use compounds of other elements, especially those of elements ofgroups 1 of 2 of the Periodic Table (e.g. Na, K, Ca and Mg) or of groups7 to 10 of the Periodic Table (e.g. Mn, Fe, Co and Ni). Preferably,however, compounds of the elements just mentioned account for not morethan 20 mol %, and in particular not more than 10 mol %, of the overallamount of hydrolysable monomeric compounds used.

With particular preference, the coating composition is a coatingcomposition obtained by the sol-gel process and based on organicallymodified inorganic compounds, particularly silane compounds.Hydrolysable silane compounds are used in particular, preferably atleast some of the hydrolysable silane compounds having at least onenon-hydrolysable substituent. By way of example, a preferred coatingcomposition comprises a polycondensate which is obtainable by thesol-gel process and is based on

-   (A) one or more silanes of the general formula (I)    R_(a)SiX_((4-a))  (I)-    in which the radicals R are identical or different and are    non-hydrolysable groups, the radicals X are identical or different    and are hydrolysable groups or hydroxyl groups and a is 0, 1, 2 or    3, a being greater than 0 for at least 40 mol % of the silanes, or    an oligomer derived therefrom, and-   (B) if desired, one or more compounds of glass-forming or    ceramic-forming elements.

In the general formula (I), the hydrolysable groups X, which may beidentical to or different from one another, are, for example, hydrogenor halogen (F, Cl, Br or I), alkoxy (preferably C₁₋₆ alkoxy, such asmethoxy, ethoxy, n-propoxy, isopropoxy and butoxy), aryloxy (preferablyC₆₋₁₀ aryloxy, such as phenoxy), acyloxy (preferably C₁₋₆ acyloxy, suchas acetoxy or propionyloxy), alkylcarbonyl (preferably C₂₋₇alkylcarbonyl, such as acetyl), amino, monoalkylamino or dialkylaminohaving preferably from 1 to 12, in particular from 1 to 6, carbon atoms.

Non-hydrolysable radicals R which may be identical to or different fromone another may be non-hydrolysable radicals R having a functional groupor without a functional group.

The non-hydrolysing radical R is, for example, alkyl (preferably C₁₋₈alkyl such as methyl, ethyl n-propyl isopropyl, n-butyl, s-butyl andt-butyl, pentyl, hexyl, octyl or cyclohexyl), alkenyl (preferably C₂₋₆alkenyl, such as vinyl, 1-propenyl, 2-propenyl and butenyl), alkynyl(preferably C₂₋₆ alkynyl such as acetylenyl and propargyl) and aryl(preferably C₆₋₁₀ aryl, such as phenyl and naphthyl). The radicals R andX may if desired have one or more customary substituents, such ashalogen or alkoxy.

Specific examples of the functional groups of the radical R are theepoxy, hydroxylether, amino, monoalkylamino, dialkylamino, amide,carboxyl, vinyl, acryloyloxy, methacryloyloxy, cyano, halogen, aldehyde,alkylcarbonyl and phosphoric acid group. These functional groups areattached to the silicon atom via alkylene, alkenylene or arylenebridging groups, which may be interrupted by oxygen groups or —NH—groups. These bridging groups are derived, for example, from theabove-mentioned alkyl, alkenyl or aryl radicals. The radicals R having afunctional group contain preferably from 1 to 18 carbon atoms, inparticular from 1 to 8 carbon atoms. Of course, the radical R may alsohave more than one functional group.

In one preferred embodiment, use is made of hydrolysable silanes havinga functional group, in particular having the abovementioned functionalgroups, preferably epoxy groups, such as a glycidyl group or glycidyloxygroup, or (meth)acryloyloxy groups. They comprise, in particular,silanes of the general formula (I) in which X is preferably C₁₋₄ alkoxyand with particular preference is methoxy and ethoxy, and R is aglycidyloxy-(C₋₆)-alkylene radical or a(meth)acryloyloxy-(C₁₋₆)-alkylene radical, in which (C₁₋₆)-alkylene is,for example, methylene, ethylene, propylene or butylene. Specificexamples of hydrolysable silanes which can be used in accordance withthe invention may be found, for example, in EP-A-195493. Owing to theirready availability, the use of γ-glycidyloxypropyltrimethoxysilane,γ-glycidyloxypropyltriethoxysilane,3-(meth)acryloyloxy-propyltriethoxysilane and3-(meth)acryloyloxypropyl-trimethoxysilane is particularly preferred inaccordance with the invention. (Meth)acryl- stands for methacryl- oracryl-.

Where use is made of abovementioned silanes having a non-hydrolysablesubstituent with an epoxy group, it is preferred to use a curingcatalyst which is selected from Lewis bases and alkoxides of titanium,zirconium or aluminium. This curing catalyst acts in particular as acatalyst for epoxide-epoxide and/or polyol-epoxide crosslinking. Thecuring catalyst is added to the corresponding compositions generally inan amount of from 0.01 to 0.6 mol per mole of epoxide group in thehydrolysable silanes. Preferred amounts are in the range from 0.02 to0.4 mol and in particular from 0.05 to 0.3 mol of curing catalyst permole of epoxide group.

The Lewis base is preferably a nitrogen compound. Nitrogen compounds ofthis kind may be selected, for example, from N heterocycles, phenolscontaining amino groups, polycyclic amines and ammonia (preferably inthe form of an aqueous solution). Specific examples include1-methylimidazole, 2-(N,N-dimethylaminomethyl)phenol, 2,4,6-tris(N,N-dimethylaminomethyl)phenol and 1,8-diazabicyclo[5.4.0]-7-undecene.Of these compounds, particular preference is given to 1-methylimidazole.Another class of nitrogen-containing Lewis bases which may be used inaccordance with the invention are hydrolysable silanes possessing atleast one non-hydrolysable radical which includes at least one primary,secondary or tertiary amino group.

The alkoxides of Ti, Zr or Al preferably comprise one such of thegeneral formula (II)M(OR′″)_(m)  (II)in which M is Ti, Zr or Al, R′″ is an alkyl group having preferably from1 to 4 carbon atoms (methyl, ethyl, n-propyl, isopropyl, n-butyl,sec-butyl or tert-butyl) or an alkyleneoxyalkyl group having preferablyfrom 1 to 4 carbon atoms for both the alkylene unit and the alkyl unit(e.g. methylene, ethylene, 1,2-propylene, 1,3-propylene and 1,4-butylenefor the alkylene unit and the examples given above for the alkyl groupfor the alkyl unit) and m is 4 (M=Ti, Zr) or 3 (M=Al). Preferred curingcatalysts are Al(OCH₂CH₂OC₄H₉), (aluminium tributoxyethoxide), in whichthe butyl group is preferably an n-butyl group, aluminium sec-butoxide,and mixtures of aluminium tributoxyethoxide and aluminium sec-butoxide.For further details, refer to DE-A-4338361.

Where abovementioned silanes having a non-hydrolysable substituenthaving a functional group are used, it is also possible to use otherhydrolysable compounds of glass-forming or ceramic-forming elementstogether with the hydrolysable silane having a functional group, theamount of the other hydrolysable compounds preferably not exceeding 80mol %, and in particular 60 mol %, based on the total amount ofhydrolysable compounds used. With preference at least 10 mol %, and withparticular preference at least 20 mol %, of all of the hydrolysablecompounds used are the other hydrolysable compounds which are differentfrom the hydrolysable silane(s) having at least one functional group ona non-hydrolysable substituent.

Particularly in the case of coating compositions based on hydrolysablesilane compounds having an epoxide group, it is possible to use, as afurther component, an organic monomer, oligomer or polymer having atleast one epoxide group, or mixtures thereof. These organic monomers,oligomers or polymers having epoxide groups are, for example, compoundsknown per se which are used in the prior art as epoxy resins, as castingresins and as epoxy reactive diluents.

In the case of the further hydrolysable compounds of glass-forming orceramic-forming elements, it is possible to use compounds of all of theglass-forming or ceramic-forming elements set out above. As examples ofthe hydrolysable groups of these compounds, reference may be made to theexamples of X set out in formula (I). Preferred examples are thecompounds of the formula (II) and the compounds H set out inDE-A-4338361. Besides the hydrolysable groups, the compounds may alsocontain non-hydrolysable groups. Except for Si, however, this is notpreferred. As examples, reference may likewise be made to the examplesof R set out in formula (I). With preference not more than 70 mol %, inparticular not more than 50 mol %, of all the hydrolysable compounds arecompounds of glass-forming or ceramic-forming elements that are not Si.

As hydrolysable compounds it is also possible to use, additionally oralone, for example, one or more hydrolysable silicon compounds having atleast one non-hydrolysable radical containing from 5 to 30 fluorineatoms attached to carbon atoms which may be separated from Si by atleast two atoms. As hydrolysable groups in this case it is possible, forexample, to use those as specified for X in formula (I). Silanes of thiskind are described in detail in DE 41 18 184. These fluorinated silanesare used, where desired, generally in an amount of from 0.1 to 15% byweight, preferably from 0.2 to 10% by weight, and with particularpreference from 0.5 to 5% by weight, based on the weight of allhydrolysable compounds.

Besides the inorganic compounds or organically modified inorganiccompounds, the coating composition may also be based on purely organiccompounds (monomers). If desired, some or all of the compound which ispolymerizable or curable thermally or photochemically to form a polymermay be replaced by a corresponding polymer. This polymer based onorganic compounds preferably still has reactive groups via which furtherpolymerization or curing may take place. Where the coating compositionis based only on these polymers based on organic compounds, it isnecessary that the reactive groups are present. The organic monomers andpolymers which can be used are, for example, the customary monomers andcoating systems known from the prior art, such as are described, forexample, in Ullmanns Encyklopädie der technischen Chemie, Vol. 15, 4thed., 1978, p. 589 ff.

Specific examples of polymerizable monomers which result in a purelyorganic polymer matrix are (meth)acrylic acid, (meth)acrylic esters,(meth)acrylonitrile, styrene and styrene derivatives, alkenes (e.g.ethylene, propylene, butene, isobutene), halogenated alkenes (e.g.tetrafluoro-ethylene, chlorotrifluoroethylene, vinyl chloride, vinylfluoride, vinylidene fluoride, vinylidene chloride), vinyl acetate,vinylpyrrolidone, vinylcarbazole, and mixtures of such monomers.Polyunsaturated monomers may also be used, examples including butadieneand ethylene dimethacrylate.

Suitable corresponding polymers include any desired known plastics, forexample polyacrylic acid, polymethacrylic acid, polyacrylates,polymethacrylates, polyolefins, polystyrene, polyamides, polyimides,polyvinyl compounds, such as polyvinyl chloride, polyvinyl alcohol,polyvinyl butyral, polyvinyl acetate and corresponding copolymers, e.g.poly(ethylene-vinyl acetate), polyesters, e.g. polyethyleneterephthalate or polydiallyl phthalate, polyarylates, polycarbonates,polyethers, e.g. polyoxymethylene, polyethylene oxide or polyphenyleneoxide, polyether ketones, polysulphones, polyepoxides andfluoropolymers, e.g. polytetrafluoroethylene. It is preferred to usetransparent polymers or corresponding monomers.

In one preferred embodiment, crosslinkable coating systems based onorganic monomers or corresponding polymers are used. These systems maybe based on the abovementioned polymers. Here again, the systems inquestion are the customary systems known from the prior art, which areset out, for example, in the abovementioned Ullmann reference. Specificexamples are acrylic resins, alkyd resins, polyester resins(crosslinking by way of amino resins, for example), polyurethane resinsand epoxy resins, and the corresponding monomer systems.

Additionally, additives known in the field of coatings technology may beadded to the coating composition based on the compounds which arepolymerizable or curable thermally or photochemically to form a polymer.Examples of such additives include solvents, crosslinking agents,lubricants, nanoscale particulate solids, polymerization initiators,photosensitizers or levelling agents. Examples of lubricants aresurfactants, fluorosilanes or graphite. For the nanoscale particulatesolids that can be used, reference may be made to the description below.

Since the application of the coating composition to this substrate isnormally carried out wet-chemically, the coating composition preferablyincludes a solvent. This solvent comprises the customary solvents whichare used in the coatings field. Examples of suitable solvents,particularly for compounds which form an organically modified inorganicmatrix, are alcohols, preferably lower aliphatic alcohols (C₁-C₈alcohols), such as methanol, ethanol, 1-propanol, isopropanol and1-butanol, ketones, preferably lower dialkyl ketones, such as acetoneand methyl isobutyl ketone, ethers, preferably lower dialkyl ethers,such as diethyl ether, or monoethers of diols, such as ethylene glycolor propylene glycol with C₁-C₈ alcohols, amides, such asdimethylformamide, and mixtures thereof. Examples of high-boilingsolvents are triethylene glycol, diethylene glycol diethyl ether andtetraethylene glycol dimethyl ether. For further solvents, particularlyfor compounds which form an organic matrix, reference may again be madeto the abovementioned Ullmann reference.

The coating composition may comprise crosslinking agents. Thecrosslinking agents contain at least two reactive groups which are ableto react with the functional groups present in the coating composition.The nature of the crosslinking agents is of course guided by thefunctional groups that are present in the coating composition. Theselection of appropriate crosslinking agents is commonplace for theperson skilled in the art. In the case of coating compositionscontaining epoxide, for example, use may be made of crosslinking agentscontaining organic or inorganic groups having reactive hydrogen, e.g.amine, isocyanate or hydroxyl groups.

Polymerization initiators which can be used are photoinitiators andthermal polymerization catalysts, which are selected as a function ofthe composition used and are known to the person skilled in the art.Examples are radical photoinitiators, radical thermoinitiators, cationicphotoinitiators, cationic thermoinitiators, and any desired combinationsthereof.

Specific examples of radical photoinitiators that can be used areIrgacure® 184 (1-hydroxycyclohexyl phenyl ketone), Irgacure® 500(1-hydroxycyclohexyl phenyl ketone, benzophenone) and otherphotoinitiators of the Irgacure® type obtainable from Ciba-Geigy;Darocur® 1173, 1116, 1398, 1174 and 1020 (available from Merck);benzophenone, 2-chlorothioxanthone, 2-methylthioxanthone, 2isopropylthioxanthone, benzoin, 4,4′-dimethoxybenzoin, benzoin ethylether, benzoin isopropyl ether, benzil dimethyl ketal,1,1,1-trichloroacetophenone, diethoxyacetophenone and dibenzosuberone.Examples of radical thermoinitiators include organic peroxides in theform of diacyl peroxides, peroxydicarbonates, alkyl peresters, alkylperoxides, perketals, ketone peroxides and alkyl hydroperoxides, andalso azo compounds. Specific examples which could be mentioned hereinclude in particular dibenzoyl peroxide, tert-butyl perbenzoate andazobisisobutyronitrile. An example of a cationic photoinitiator isCyracure® UVI-6974, while a preferred cationic thermoinitiator is1-methylimidazole.

Photochemical curing may take place in accordance with customarytechniques, for example by means of WV radiation. In addition it is alsopossible to carry out further customary curing techniques, such aselectron beam curing and laser curing.

These polymerization initiators are used in the customary amounts knownto the person skilled in the art (e.g. 0.01-5% by weight, in particular0.1-2% by weight, based on the overall solids content of the coatingcomposition). It is of course also possible to operate without apolymerization initiator if this initiator is unnecessary.

In one preferred embodiment, the coating composition is obtained fromhydrolysable compounds by the sol-gel process. In this process thehydrolysable compounds are hydrolysed with water, where appropriate byheating or by means of acid or base catalysis, and are partly condensed.Either stoichiometric amounts of water or else larger or smaller amountsmay be used. The sol which forms may readily be adjusted to theviscosity that is desired for the coating composition, this being doneby the person skilled in the art using appropriate parameters, such asdegree of condensation, solvent or pH. The coating composition ispreferably used in the form of a sol for the coating. Further details ofthe sol-gel process may be found, for example, in W. Noll, Chemie undTechnologie der Silicone, 2nd ed., Verlag Chemie, 1968.

Coating compositions which can be used with preference can be found, forexample, in EP-A-0 607 213 or in DE-A-4338361, which is herebyincorporated in its entirety by reference.

The nanostructured topcoat comprises nanophases, in the form for exampleof nanoscale sol particles and/or particulate solids. The particles inquestion are, in particular, nanoscale inorganic sol particles and/orparticulate solids, with or without surface modification.

The nanoscale sol particles and/or particulate solids comprise particleshaving an average size (an average particle diameter) of not more than1000 nm, preferably not more than 200 nm, more preferably not more than100 nm, and in particular not more than 70 nm. A particularly preferredparticle size range is situated from 1 to 100 nm, in particular from 5to 50 nm

The nanoscale (inorganic) sol particles and/or particulate solids mayconsist of any desired materials, but preferably consist of metals and,in particular, of metal compounds such as, for example, (anhydrous orhydrated) oxides such as ZnO, CdO, SiO₂, TiO₂, ZrO₂, CeO₂, SnO₂, Al₂O₃,In₂O₃, La₂O₃, Fe₂O₃, Cu₂O, Ta₂O₅, Nb₂O₅, V₂O₅, MoO₃ or WO₃;chalkogenides such as, for example, sulphides (e.g. CdS, ZnS, PbS andAg₂S), selenides (e.g. GaSe, CdSe and ZnSe) and tellurides (e.g. ZnTe orCdTe), halides such as AgCl, AgBr, AgI, CuCl, CuBr, CdI₂ and PbI₂;carbides such as CdC₂ or SiC; arsenides such as AlAs, GaAs and GeAs;antimonides such as InSb; nitrides such as BN, AlN, Si₃N₄ and Ti₃N₄;phosphides such as GaP, InP, Zn₃P₂ and Cd₃P₂; phosphates, silicates,zirconates, aluminates, stannates, and the corresponding mixed oxides(for example indium-tin oxides (ITO) and those with perovskite structuresuch as BaTiO₃ and PbTiO₃).

The nanoscale inorganic sol particles and/or particulate solids used arepreferably those of oxides, sulphides, selenides and tellurides ofmetals and mixtures thereof. Particular preference is given inaccordance with the invention to nanoscale particles of Si₂, TiO₂, ZrO₂,ZnO, Ta₂O₂, SnO₂ and Al₂O₃ (in all modifications, particularly asboehmite, AlO(OH)), and mixtures thereof.

The nanoscale particulate solids used in accordance with the inventionmay be prepared in a conventional manner, for example by flamepyrolysis, plasma processes, gas phase condensation processes, colloidtechniques, precipitation processes, sol-gel processes, controllednucleation and growth processes, MOCVD processes and (micro)emulsionprocesses. These processes are described at length in the literature. Inparticular, use may be made, for example, of metals (for example, afterreduction, including precipitation processes), ceramic oxide systems (byprecipitation from solution), or else saltlike systems or multicomponentsystems. The saltlike or multicomponent systems also includesemiconductor systems.

The nanoscale sol particles and/or particulate solids may be used asthey are or in surface-modified form. Preference is generally given tousing nanoscale sol particles and/or particulate solids which have beenprovided with addition-polymerizable/polycondensable groups; in the caseof nanoscale sol particles and/or particulate solids comprising SiO₂, inparticular, however, it is also possible to achieve very good resultswithout surface modification. For this purpose it is possible, forexample, to use commercial silica products, examples being silica sols,such as the Levasils® from Bayer AG, or pyrogenic silicas, such as theAerosil products from Degussa. The particles which are not surfacemodified may also, however, be prepared in situ.

The nanoscale inorganic particles that may be used in accordance withthe invention and are provided with addition-polymerizable and/orpolycondensable organic surface groups may be prepared in principle bytwo different methods: firstly, by surface modification of pre-preparednanoscale inorganic particles, and secondly by preparation of theseinorganic nanoscale particles using one or more compounds which possessaddition-polymerizable and/or polycondensable groups of this kind. Thesetwo methods are described in greater detail later on below.

The organic addition-polymerizable and/or polycondensable surface groupsmay be any desired groups which are known to the person skilled in theart and which are able to undergo radical, cationic or anionic, thermalor photochemical addition polymerization or thermal or photochemicalpolycondensation (where appropriate in the presence of a suitableinitiator and/or catalyst). Preference is given in accordance with theinvention to surface groups which possess a (meth)acryloyl, allyl, vinylor epoxy group, particular preference being given to (meth)acryloyl andepoxy groups. In the case of the polycondensable groups, mention couldbe made in particular of hydroxyl, carboxyl and amino groups, by meansof which ether, ester and amide bonds can be obtained.

It is also preferred in accordance with the invention for the organicgroups present on the surfaces of the nanoscale particles and containingthe addition-polymerizable and/or polycondensable groups to have arelatively low molecular weight. In particular, the molecular weight ofthe (purely organic) groups should not exceed 500 and preferably 300,with particular preference 200. Of course, this does not rule out thecompounds (molecules) containing these groups having a much highermolecular weight, of course (for example, 1000 or more).

As already mentioned above, the addition-polymerizable/polycondensablesurface groups may in principle be provided by two methods. Wheresurface modification of pre-prepared nanoscale particles is carried out,compounds suitable for this purpose are all (preferably low molecularweight) compounds which on the one hand possess one or more groups whichare able to react or at least interact with (functional) groups (such asOH groups in the case of oxides, for example) present on the surface ofthe nanoscale particulate solids, and secondly have at least oneaddition-polymerizable/polycondensable group. Accordingly, thecorresponding compounds may, for example, form both covalent and ionic(saltlike) or coordinative (complex) bonds to the surface of thenanoscale particulate solids, while pure interactions that may bementioned include, by way of example, dipol—dipol interactions, hydrogenbonding, and van der Waals interactions. Preference is given to theformation of covalent and/or coordinative bonds. Specific examples oforganic compounds which can be used for surface modification of thenanoscale inorganic particulate solids include, for example, unsaturatedcarboxylic acids such as acrylic acid and methacrylic acid, β-dicarbonylcompounds (e.g. β-diketones or β-carbonylcarboxylic acids) havingpolymerizable double bonds, ethylenically unsaturated alcohols andamines, epoxides and the like. Particularly preferred such compounds inaccordance with the invention are—especially in the case of oxideparticles—hydrolytically condensable silanes having at least (andpreferably) one non-hydrolysable radical which possesses a polymerizablecarbon-carbon double bond or an epoxide ring. Silanes of this kindpreferably have the general formula (III):Y—R′—SiR² ₃  (II)in which Y is CH₂═CR³—COO, CH₂═CH or glycidyloxy, R³ is hydrogen ormethyl, R¹ is a divalent hydrocarbon radical having from 1 to 10,preferably from 1 to 6, carbon atoms, containing if desired one or moreheteroatom groups (e.g. O, S, NH) which separates adjacent carbon atomsfrom one another, and the radicals R², identical to or different fromone another, are the groups specified for X in the general formula (I)and arc selected in particular from alkoxy, aryloxy, acyloxy andalkylcarbonyl groups and also halogen atoms, (especially F, Cl and/orBr).

The groups R² are preferably identical and are selected from halogenatoms, C₁₋₄ alkoxy groups (e.g. methoxy, ethoxy, n-propoxy, isopropoxyand butoxy), C₆₋₁₀ aryloxy groups (e.g. phenoxy), C₁₋₄ acyloxy groups(e.g. acetoxy and propionyloxy) and C₂₋₁₀ alkylcarbonyl groups (e.g.acetyl. Particularly preferred radicals R² are C₁₋₄ alkoxy groups andespecially methoxy and ethoxy.

The radical R¹ is preferably an alkylene group, especially one havingfrom 1 to 6 carbon atoms, such as ethylene, propylene, butylene andhexylene. If Y is CH₂═CH, R¹ is preferably methylene and in this casemay also be a simple bond.

Y is preferably CH₂═CR³—COO (with R³ preferably being CH₃) orglycidyloxy. Accordingly, particularly preferred silanes of the generalformula (I) are (meth)acryloyloxyalkyltrialkoxysilanes such as3-methacryloyloxypropyltri(m)ethoxysilane andglycidyloxyalkyltrialkoxysilanes such as 3-glycidyloxypropyltri(m)ethoxysilane.

If the nanoscale inorganic particles have actually been prepared usingone or more compounds which possessaddition-polymerizable/polycondensable groups, there is no need forsubsequent surface modification (although this is of course possible asan additional measure).

The in situ preparation of nanoscale inorganic sol particles and/orparticulate solids having addition-polymerizable/polycondensable surfacegroups will be elucidated below taking SiO₂ particles as an example. Forthis purpose, the SiO₂ particles may be prepared, for example, by thesol-gel process using at least one hydrolytically polycondensable silanehaving at least one addition-polymerizable/polycondensable group.Examples of suitable silanes of this kind are the above-describedsilanes of the general formula (I) without non-hydrolysablesubstituents. In this case it is also possible to use silanes whichpossess a (non-hydrolysable) hydrocarbon group without any functionalgroup, such as methyl- or phenyl-trialkoxysilanes. Especially when aneasy-to-clean surface of the coat is desired, it may be advisable to usea certain amount (for example, up to 60 mol % and in particular up to 50mol %, on the basis of all silanes used) of abovementioned silaneshaving fluorine-containing (non-hydrolysable) radicals.

An (additional) constituent of the composition comprising nanoscale solparticles and/or particulate solids may also, for example, be at leastone monomeric or oligomeric species which possesses at least one groupwhich is able to react (by addition polymerization or polycondensation)with the addition-polymerizable/polycondensable groups that are presenton the surface of the nanoscale particles. Examples of such species thatmay be mentioned include monomers having a polymerizable double bond,such as acrylates, methacrylates, styrene, vinyl acetate and vinylchloride, for example. For further details of the nanoscale particulatesolids, the monomeric or oligomeric species, and additives which can beused additionally, refer to DE-A-19746885, the content of which ishereby incorporated in its entirety by reference. For the compositioncomprising nanoscale particulate solids as well it is possible inparticular to use the additives set out above for the coatingcomposition. The specific examples set out there for the additives mayalso be used for the topcoat.

The composition comprising nanoscale particles is applied preferably bywet-chemical means to the basecoat. The composition is thereforepreferably in the form of a sol or in the form of a composition(suspension) which is still flowable. The liquid constituent of thiscomposition is composed, for example, of water and/or (preferablywater-miscible) organic solvents arid/or compounds which were used orproduced in the preparation of the nanoscale particles or their surfacemodification (for example, alcohols in the case of alkoxysilanes).Suitable organic solvents which may be used in addition are, forexample, alcohols, ethers, ketones, esters, amides and the like. Forthese, reference may be made to the solvents specified above. Besidesthe solvent, the composition comprising nanoscale particles in onepreferred embodiment further comprises no further additives apart frompolymerization initiators whose use may be intended and which haslikewise already been set out above.

The substrate to be coated may comprise, for example, a substrate madeof metal, including non-ferrous metal, glass, ceramic, glass ceramic,plastic, wood or paper. The substrate may be in any desired form, forexample as a plate, foil, disc or irregular form. Since a particularadvantage of the present invention is that abrasion-resistant diffusionbarrier coats can be obtained without a need to use high temperatures,the invention is of course particularly suitable for thermally sensitivesubstrates. These include substrates of plastic in particular. Examplesof plastics substrates are polyethylene, polypropylene, polyacrylate,such as polymethyl methacrylate and polymethyl acrylate, polyvinylbutyral, polycarbonate, polyurethanes, ABS copolymers or polyvinylchloride. Since the coating systems of the invention can also easily beprepared in transparent form, preference is given to transparentsubstrates, especially plastic. The coating system may also of course beused for substrates which are not thermally sensitive.

The substrate may be pretreated conventionally, for example, to achievecleaning, degreasing, corrosion protection, smoothing, or betteradhesion to the coating. The substrate may be provided, for example,with an undercoat or may be pretreated with a customary primer, such assilanes or aminosilanes, or pretreated by means of Ar/O₂ plasma orcorona discharge, or appropriate irradiation techniques.

Both the coating composition for the basecoat and the composition forthe topcoat(s) are applied to the substrate preferably by wet-chemicalmeans, particularly in the form of a sol. They can be applied in anycustomary manner, for example by squirting, spraying, flowcoating,brushing, electrocoating, dipping, spincoating or flooding. The basecoatpreferably has a dry film thickness in the range of 1-50 μm, preferably3-30 μm, and in particular 5-10 μm. The topcoat, or each topcoat,preferably has a dry film thickness in the range of 100-1000 nm,preferably 150-500 nm, and in particular 200-300 nm.

The coating composition for the basecoat is selected so that a hardbasecoat is formed. The hardness of coatings can be determined invarious ways, by scratch testing, for example. Standardized methods areindicated, for example, in the abovementioned Ullmann reference. A hardbasecoat is understood here to be preferably a basecoat which has atleast the same hardness and preferably a greater hardness than thesubstrate to be coated.

Following application, the coating composition for the basecoat isexposed to conditions in which, although drying and/or complete orpartial polymerization or curing can take place, the operation isnevertheless such that the basecoat still contains reactive surfacegroups.

In the case of a partial polymerization and/or curing of the basecoat,the coating composition, after flashing off, for instance, may betreated thermally or photolytically, for example, in order to achieveincipient drying and/or incipient curing. The conditions, such astemperature, amount of radiation or duration of the treatment, however,should be chosen such that the basecoat still contains reactive groups.This can be achieved by, for example, drying the applied basecoat at atemperature which lies within a range from room temperature up to notmore than 100° C., preferably not more than 85° C., in particular notmore than 70° C.

In the case of complete polymerization and/or curing of the basecoat,the basecoat is aftertreated in order to generate reactive surfacegroups, by means of flame, plasma, corona, oxidation or reductiontreatment or primer coating, for example.

The reactive groups are groups by means of which fisher polymerizationor curing is possible. Regarding these reactive groups, reference ismade to the functional groups specified in respect of the materials forthe coatings. These reactive groups also include, in particular, thehydrolysable groups (e.g. M-Oalkyl, M-glass-forming or ceramic-formingelements such as Si) that are still present in the coating compositionsbased on the inorganic compounds or organically modified compounds, andthe hydroxyl groups (e.g. M—OH, M-glass-forming or ceramic-formingelements such as Si) which result following hydrolysis and have not yetbeen condensed to form, for example, siloxane groups. By way of thesegroups, then, it is possible for further condensation to take place.Examples of reactive groups present with preference are hydroxyl groups,hydrolysable groups on glass-forming or ceramic-forming elements (e.g.M-Oalkyl, M—OH), epoxide groups and (meth)acryloyloxy groups. As aresult of the reactive groups, there is also sufficient reactivitypresent for achieving sufficient adhesion of the topcoat.

A top the basecoat that still contains reactive surface groups, then,the nanostructured topcoat is applied and is subsequently cured orheat-treated. Curing may take place, for example, thermally orphotochemically. For the possible curing methods, reference is made tothe methods described in connection with the basecoat. It is assumedthat there are also crosslinking reactions between the nanoscaleparticles by way of any addition-polymerizable/polycondensable surfacegroups that are present. Particularly when using nanoscale particleswithout addition-polymerizable/polycondensable surface groups, a heattreatment is carried out. Without being bound to any one theory, it isassumed that linking reactions (for example, by way of remaining silanolgroups) or densification reactions take place. In the case of heattreatment it is of course also possible for curing reactions to takeplace.

The thermal curing or heat treatment takes place, for example, attemperatures of not more than 200° C., preferably from 60 to 160° C.,with particular preference from 120 to 130° C. Possible temperatureranges, therefore, are well below the temperatures which are normallyconsidered necessary for densification or sintering (usually at least450° C.). Despite this, extremely abrasion-resistant diffusion barriercoats are obtained. This is all the more surprising on account of thefact that direct application of compositions comprising nanoscaleparticles to a substrate does not afford the possibility of producingsuitable coatings. It is supposed that a role is also played byinteractions between the remaining reactive groups of the coatingcomposition and the reactive groups in the composition used for thetopcoat, by these groups leading, for example, to adhesion-promotingbonds between the coats.

It has been found that the permeation rate of gases is significantlyreduced. The abrasion values found after 1000 cycles with a TaberAbraser, measured as the diffuse light loss in %, are in some cases notmore than 1%. Compared with this, glass gives diffuse light losses of1.5%, transparent plastics 30-60%, and customary hard coats 3-20%.

The abrasion-resistant diffusion barrier coat system of the invention onthe substrate is suitable as a protective coat for any desiredsubstrates. Fields of use include coatings for machines, floors,components, instruments, rotors, articles of everyday use, operatingelements, glass, materials of transparent plastic, glazing, displays,drinking vessels, non-ferrous metals, furniture, jewellery, and also invehicle construction and in interior fitting.

EXAMPLES Example 1 Preparation of an SiO₂ Sol for the Topcoat

98.87 g of tetraethoxysilane (TEOS) were mixed with 63.83 g of ethanolto give a solution A. Additionally, 63.83 g of ethanol, 72.50 g ofdeionized water and 1.38 g of HCl (37%) were mixed to give a solution B.Mixing of solutions A and B with warming at 30-40° C. produced a solwhich was stirred at room temperature for 1.5 h and then stored at −20°C.

Immediately prior to application, the sol was diluted with ethanol to asolids content of 3% by weight.

Example 2 Preparation of a Surface-Modified SiO₂ Sol for the Topcoat

To prepare an alcoholic 5.1% strength SiO₂ sol, 247 g oftetraethoxysilane (TEOS) were hydrolysed and condensed with 76 g ofethanol in HCl-acidic solution (76 g of ethanol+76 g of double-distilledH₂O+5.8 g of HCl, 37% in water). Glycidyloxypropyltrimethoxysilane(GPTS) was added in an SiO₂:GPTS weight ratio of 4:1 and the sol wasstirred at 50° C. for 5 h. As an alternative toglycidyloxypropyltrimethoxysilane, it is possible to usemethacryloyloxypropyltrimethoxysilane (MPTS) in a corresponding weightratio.

Example 3 Preparation of an SiO₂ Sol for the Topcoat

To prepare a 3% by weight SiO₂ sol, 1.95 g of silica sol (Levasil200S/30 from Bayer), 43.35 g of ethanol and 3.00 g of tetraethoxysilane(REOS) were mixed and stirred at room temperature for 18 h.

Example 4 Preparation of a CeO₂/SiO₂ Sol for the Topcoat

To prepare a 10% by weight CeO₂/SiO₂ sol, 25 g of ethanol, 10.0 g of a20% by weight cerium oxide suspension and 5.0 g of tetraethoxysilane(TEOS) were mixed and stirred at room temperature for 24 h.

Example 5 Preparation of a TiO₂ Sol for the Topcoat

For preparing a nanoparticulate TiO₂ sol, 2.1 g of tetraisopropylorthotitanate were added to a mixture of isopropanol, 0.981 g ofconcentrated HCl (37% by weight in water) and 0.105 g of H₂O and themixture was stirred at 25° C. for 24 h. Then 2 g of MPTS were added to200 g of TiO₂ sol and the mixture was stirred under reflux at 50° C. for5 h. A fraction of the isopropanol (10 g) was distilled off underreduced pressure and 14 g of 2-isopropoxyethanol and the photoinitiator,UVI® 6974 (Union Carbide), were added. As an alternative to MPTS, it ispossible to use the same amount of GPTS.

Example 6 Preparation of the Coating System

Application of the Primer Solution

PC plates (Makrolon 3103) measuring 10×10 cm² were used as thesubstrate. The primer solution (2% by weight ofγ-aminopropyltriethoxysilane in isopropanol) was applied by spincoating(conditions: application volume: 3 ml; spinning speed: 1500 rpm;acceleration: 05; duration: 10 s). Curing was carried out at 130° C. (30minutes) in a forced air drying oven.

Application of the Hard Basecoat

Following the application of the primer, a hard coating system based onhydrolysable epoxysilanes was applied again by spincoating (conditions:application volume: 4 ml; spinning speed: 600 rpm; acceleration: 05;duration: 10 s). This was followed by incipient curing of this basecoatat 80° C. (5 minutes) in a forced air drying oven.

Application of the Topcoat

After the basecoat had been applied, a coating sol for the topcoat wasapplied, again by means of spincoating (conditions: application volume:3 ml; spinning speed: 1500 rpm; acceleration: 05; duration: 10 s) Thiswas followed by curing. Curing was carried out at 130° C. (2 h) in aforced air drying oven.

Characterization of the Coats

Coat systems are obtained whose adhesion according to cross-cut/tapetest (DIN 53151 and DIN 58196-K2 respectively) is very good (GT/TT=0/0).The diffuse light loss after 1000 cycles of the Taber Abtaser test PIN52347/CS-10F abrading wheels/load: 2×500 g/height of suction tube: 3 mm)is between 1 and 3%. (These values relate to different samples preparedby different individuals on different days in order to providestatistical reliability.) The film thickness (dry) of the applied hardcoat is about 5 μm. The film thickness (dry) of the topcoats, measuredusing a profilometer, is from approximately 200 to 300 nm.

The diffusion rates were measured using a Permatran-W 3/31 from Mocon at25° C. and 100% relative atmospheric humidity. The diffusion rates ofwater vapour are in some cases up to 20% below the diffusion rates ofthe uncoated films.

1. A substrate having an abrasion-resistant diffusion barrier coatsystem, wherein the coat system comprises: (a) a hard basecoatcomprising a coating composition based on compounds which have beenpolymerized or cured thermally or photochemically to form a polymer; (b)a nanostructured topcoat obtainable by applying a composition comprisingat least one of nanoscale sol particles and particulate solids to thebasecoat while the basecoat comprises reactive surface groups, and thencarrying out at least one of a heat treatment and a curing operation. 2.The substrate of claim 1, wherein the basecoat has a dry film thicknessof from 1 μm to 50 μm.
 3. The substrate of claim 1, wherein the coatsystem further comprises one or more additional nanostructured topcoats.4. The substrate of claim 1, wherein the topcoat has a dry filmthickness of from 100 nm to 1000 nm.
 5. The substrate of claim 3,wherein each topcoat has a dry film thickness of from 100 nm to 1000 nm.6. The substrate of claim 1, wherein the topcoat has been prepared fromat least one of nanoscale sol particles and particulate solids having atleast one of polymerizable and polycondensable surface groups.
 7. Thesubstrate of claim 1, wherein the at least one of nanoscale solparticles and particulate solids have an average particle size of notmore than 200 nm.
 8. The substrate of claim 2, wherein the at least oneof nanoscale sol particles and particulate solids have an averageparticle size of from 1 nm to 70 nm.
 9. The substrate of claim 4,wherein the at least one of nanoscale sol particles and particulatesolids have an average particle size of from 5 nm to 50 nm.
 10. Thesubstrate of claim 1, wherein the at least one of nanoscale solparticles and particulate solids comprise metal compounds.
 11. Thesubstrate of claim 10, wherein the metal compounds comprise at least oneof oxides, sulfides, selenides and tellurides.
 12. The substrate ofclaim 11, wherein the metal compounds comprise at least one of SiO₂,TiO₂, ZrO₂, ZnO, Ta₂O₅, SnO₂ and Al₂O₃.
 13. The substrate of claim 1,wherein the coating composition of the basecoat comprises apolycondensate obtained by a sol-gel process and derived from at leastone silane which comprises an epoxide group on a non-hydrolyzablesubstituent.
 14. The substrate of claim 13, wherein the coatingcomposition of the basecoat further comprises a curing catalyst selectedfrom at least one of Lewis bases and alkoxides of titanium, zirconiumand aluminum.
 15. The substrate of claim 1, wherein the substratecomprises a plastic material.
 16. The substrate of claim 15, wherein thesubstrate comprises at least one of polyethylene, polypropylene,polyacrylate, polyvinyl butyral, polycarbonate, polyurethane,acrylonitrile-butadiene-styrene terpolymer and polyvinyl chloride.
 17. Asubstrate having an abrasion-resistant diffusion barrier coat system,wherein the coat system comprises: (a) a hard basecoat comprising acoating composition based on compounds which have been polymerized orcured thermally or photochemically to form a polymer; (b) ananostructured topcoat obtained by applying a composition comprising atleast one of nanoscale sol particles and particulate solids to thebasecoat while the basecoat still comprises reactive surface groups, andthen carrying out at least one of a heat treatment and a curingoperation; the coating composition of the basecoat comprising apolycondensate obtained by a sol-gel process and derived from: (A) oneor more silanes of formula R_(a)SiX_((4-a)) where each R independentlyrepresents a non-hydrolyzable group; each X independently represents ahydroxy group or a hydrolyzable group; and a is 0, 1, 2 or 3 and isgreater than 0 for at least 40 mol % of the one or more silanes; or anoligomer derived therefrom; (B) optionally, one or more compounds of atleast one of glass-forming elements and ceramic-forming elements. 18.The substrate of claim 17, wherein the basecoat has a dry film thicknessof from 1 μm to 50 μm; the topcoat has a dry film thickness of from 100nm to 1000 nm, and said at least one of nanoscale sol particles andparticulate solids have an average particle size of from 5 nm to 50 nmand comprise at least one of a metal oxide, sulfide, selenide andtelluride.
 19. A process for producing a substrate provided with anabrasion-resistant diffusion barrier coat system, comprising: (a)applying to the substrate a coating composition based on compounds whichare thermally or photochemically polymerizable or curable to form apolymer; (b) at least one of curing and polymerizing the composition toform a coating which comprises reactive surface groups, (c) applying acomposition comprising at least one of nanoscale sol particles andparticulate solids to the coating of (b); (d) at least one of curing andheat-treating the composition of (c) to form a topcoat.
 20. The processof claim 19, wherein in (b) the at least one of curing and polymerizingof the composition is carried out under conditions which result in acoating which comprises reactive surface groups.
 21. The process ofclaim 19, wherein in (b) the at least one of curing and polymerizing ofthe composition is followed by a post-treatment to create reactivesurface groups on the coating.
 22. The process of claim 20, wherein in(b) the at least one of curing and polymerizing of the composition isfollowed by a post-treatment to create further reactive surface groupson the coating.
 23. The process of claim 19, wherein prior to (a), thesubstrate is treated with a primer.
 24. The process of claim 19, whereinthe coating of (b) is dried at a temperature of not higher than 100° C.before applying the composition of (c).
 25. The process of claim 19,wherein (d) comprises photochemically curing the composition of (c). 26.The process of claim 19, wherein (d) comprises thermally curing thecomposition of (c).
 27. The process of claim 24, wherein the substratecomprises a plastic substrate.
 28. The process of claim 27, wherein theat least one of nanoscale sol particles and particulate solids have anaverage particle size of not more than 200 nm.
 29. The process of claim19, wherein the at least one of nanoscale sol particles and particulatesolids have an average particle size of from 1 nm to 70 nm.
 30. Theprocess of claim 28, wherein the at least one of nanoscale sol particlesand particulate solids comprise metal compounds.
 31. The process ofclaim 30, wherein the metal compounds comprise at least one oxides,sulfides, selenides and tellurides.
 32. The process of claim 19, whereinthe at least one of nanoscale sol particles particulate solids compriseat least one of SiO₂, TiO₂, ZrO₂, ZnO, Ta₂O₅, SnO₂ and Al₂O₃.